1. Field of the Invention
The present invention relates to an apparatus for monitoring optical signal quality, and more particularly to such a quality monitoring apparatus applicable to NRZ (non-return-to-zero) optical signals.
2. Description of the Background Art
In order to estimate either the quality of an incoming transmission signal or the factor leading to its degradation, it is necessary to monitor the waveform of the signal. The waveform of an optical signal can be monitored using a sampling oscilloscope or other means. Generally, sampling oscilloscopes are complex and expensive instruments.
Such a problem is addressed by, for example, Z. Pan, et al., “Chromatic dispersion monitoring and automated compensation for NRZ and RZ data using clock regeneration fading without adding signaling”, Optical Fiber Communication Conference and Exhibit (OFC2001), WH5, March 2001. In the solution disclosed by Z. Pan, et al., frequency components corresponding to the signal bit rate are detected, and the degree of degradation of the signal is detected from an increase or decrease in the intensity. Also, U.S. Patent Application Publication No. US 2009/0016712 A1 to Kagawa proposes a method of detecting a directly undetectable frequency, by modulating a monitored light signal with, for example, a sinusoidal wave obtained by adding an offset frequency to the quarter (¼) frequency and detecting the frequency of interest at the intensity of the output beat signal.
The prior art solution presented by Pan, et al., aims to measure wavelength dispersion on transmission paths. Signal degradation is caused not by dispersion alone but it is also necessary to take account of deterioration of optical signal-to-noise ratios (SNRs). In this respect, the quality monitor functions unsatisfactorily.
According to the prior art solution taught by Kagawa, frequency components of signals are decreased by deterioration of the optical SNRs and, therefore, if only the intensities of the frequency components are detected, it is then impossible to know the cause of degradation of the actually incoming transmission signal. Furthermore, Kagawa makes a mention of only deterioration of RZ (return-to-zero) signals. By contrast, with respect to NRZ (non-return-to-zero) signals, if the dispersion increases as shown in FIGS. 1A, 1B and 1C, the line spectrum of 10 GHz does not show a sufficient intensity as a bit rate frequency component compared with the level of the modulated signal. Therefore, if the prior art solution taught by Kagawa is applied to NRZ signals, the beat signals cannot be extracted.
FIG. 1A shows a temporal waveform obtained when the dispersion is 320 ps/nm. FIG. 1B shows a spectrum derived by converting an optical signal into an electric signal without modulation. FIG. 1C shows a spectrum obtained by converting an optical signal modulated at 10.25 GHz into an electric signal.